(1) Field of the Invention
The present invention generally relates to a data reproduction system applicable to an optical disk unit such as a magnetooptical disk unit, and more particularly to a data reproduction system for generating data signals corresponding to recorded data from signal wave forms reproduced from an optical disk in which data has been recorded based on a partial response characteristic.
(2) Description of the Related Art
In an external memory unit, for example a magnetooptical disk unit, data recording/reproducing in accordance with a partial response maximum likelihood (PRML) method has been proposed to increase the capacity. In data recording in accordance with the partial response characteristic, the intersymbol interference is positively utilized, and recording data is coded in accordance with patterns of predetermined inter-symbol interference. In maximum likelihood sequence estimation (MLSE), a reproduced signal is decoded so that a signal value having a strongest likelihood is obtained from among a plurality of signal values.
An essential structure of a magnetooptical disk unit in which the data recording/reproducing is performed in accordance with the partial response maximum likelihood method is shown in FIG. 1. In FIG. 1, parts not related to the data recording/reproducing are omitted therefrom.
Referring to FIG. 1, a magnetooptical disk 4 which is a recording medium is rotated at a predetermined speed by a spindle motor 6. A magnetooptical head assembly 3 and a magnetic field generating unit 5 are provided adjacent to the magnetooptical disk 4. The magnetooptical head assembly 3 includes a laser diode which is an optical source. The magnetic field generating unit 5 generates a magnetic field in a direction perpendicular to a surface of the magnetooptical disk 4. A recording system for data has a precoder 1 and a driving circuit 2. The pre-coder 1 codes recording data supplied from a control unit (not shown) in accordance with a partial response characteristic. The driving circuit 2 drives the laser diode of the magnetooptical head assembly 3 based on the coded data from the precoder 1. Due to a heat function of the laser beam from the laser diode and the magnetic field generated by the magnetic field generating unit 5, recording marks arranged in accordance with magnetization patterns corresponding to the coded data are formed on the magnetooptical disk 4. A reproducing system for data has a PLL (Phase Locked Loop) circuit 7, a data PLL circuit 8, a delay circuit 9 and a maximum likelihood data detecting circuit 10. Servo pits (wobbled pits) are formed on the magnetooptical disk 4. In the case of reading data, the magnetooptical head assembly outputs a periodic signal (a wobbled pit signal) corresponding to the laser beam reflected by the servo pits. In the PLL circuit 7, a phase synchronization between a reference clock signal and the wobbled pit signal is accomplished, so that a clock signal is generated. This clock signal is further processed by the data PLL circuit 8 and the delay circuit 9 and supplied to the maximum likelihood data detecting circuit 10. The maximum likelihood data detecting circuit 10 operates in synchronism with the above clock signal, and reproduces data signals corresponding to edges of data recording marks from reproduced signal waveforms obtained from the magnetooptical head assembly 3. In the maximum likelihood data detecting circuit 10, a transition path for data values having the maximum likelihood is decided based on signal values sampled in synchronism with the clock signal, and signal values to be reproduced based on the transition path are decided.
Due to sensitivity variation of the magnetooptical disks, circumstantial temperature variation in a data recording process, and the variation of the optical point control, the size (the length in a direction in which a track extends) of a mark formed on the magnetooptical disk is varied. When the size of the mark is varied, the reproduced signal waveform obtained from the magnetooptical disk is varied. Particularly, since data signals corresponding to the edges of data recording marks are reproduced by the maximum likelihood data detecting circuit 10, in a reproduction data signal corresponding to front edges of the recording marks and a reproduction data signal corresponding to tail edges of the recording marks, directions in which phases of both the reproduction data signals are varied along with the variation of the size of each recording mark differ from each other. Thus, it is difficult to reproduce actual data signals by using an external clock signal without accounting for an influence of the variation of the size of each recording mark.